Users of iOS 11 and later will automatically have photos stored in HEIF instead of JPEG. What is HEIF and why did Apple switch from JPEG to HEIF for images?
HEIF stands for High Efficiency Image File Format (HEIF). Using the HEIF specification for photos on the iPhone, iPad and iPod touch saves storage space without degrading image quality.
This means more photos can be stored on an iOS device in less space. The HEIF specification was developed by the Moving Picture Experts Group (MPEG). MPEG claims that twice as much information can be stored using HEIF compared to a JPEG of the same size, with improved quality.
HEIF offers other benefits, such as including multiple images in a single file and non-destructive editing operations. More detailed information can be found in ISO/IEC 23008 - High efficiency coding and media delivery in heterogeneous environments.
Image files saved in high-efficiency use .HEIC (instead of .JPG) in the filename extension.